X-ray backscatter imaging is a powerful tool for visualizing objects located behind optically opaque barriers when the object to be imaged is only accessible from one side. It is frequently used in security applications for screening packages, luggage, vehicles and people.
X-rays or other types of high energy penetrating radiation (e.g., gamma rays) are frequently used to investigate objects or structures located behind optically opaque barriers or embedded within opaque media. When both sides of the region of interest are accessible, including the object to be investigated, investigation can be performed using transmission imaging or radiography. An example of this type of imaging is a standard medical x-ray to investigate structures inside a human body.
When only one side of the region of interest is accessible, x-ray backscatter imaging can be used to create an image of the objects behind the opaque barrier. This technique is sometimes used for security screening of people and inspection of shipping containers.
X-ray backscatter imaging is most effective when the opaque barrier is relatively thin and the object to be imaged scatters strongly, as when searching for explosives in a vehicle. Backscatter imaging also works well when the object of interest scatters weakly but is set against a background that scatters strongly, as when searching for metal weapons concealed beneath clothing on a human. Backscatter imaging fails when the barrier is too thick and does not allow a sufficient number of x-rays to pass through.
In some cases, the object of interest is not simply situated in air behind an opaque barrier, but instead immersed in an opaque medium. For example, in an oil well the well fluids (e.g., hydrocarbons, drilling fluids, etc.) are generally opaque or of poor optical quality, thus seeing objects or parts of the well through these fluids is difficult. Another example is searching for objects buried in the ground, such as landmines or improvised explosive devices. In these situations, the medium is effectively a barrier and, if x-rays can penetrate the medium material, x-ray backscatter imaging can allow one to create an image of the object.
However, if the medium scatters too strongly, then the signal from the medium itself can be greater than the signal from the object. This reduces contrast in the image and obscures detail on the object. Various image processing techniques can be applied to mitigate the influence of the medium scattering and obtain acceptable images. In a very strongly scattering medium, the signal from the medium may completely overwhelm the signal from the object. In that case, scattering from the medium must be suppressed through specific design of the imaging apparatus. Alternately, the large signal from the medium can be embraced and used to determine the distance to the surface of the object.
For each of these modes, the prior art teaches only limited methods of investigation and relatively unsophisticated apparatus to practice those methods, but in all such cases the resulting images or data provide little to no information about the distance to the target object or its depth within the opaque medium.
Furthermore, radiography techniques require access to both sides of the region of interest as the radiation source and detector must be positioned such that transmission through both the object and the medium can be measured. Backscattering techniques avoid this limitation, but will fail or be of only limited use when the medium scatters very strongly, as scattering from the medium will overwhelm the scattering from the object, thereby leading to poor image quality.
For obtaining three-dimensional representations of materials or locating objects within a medium, x-ray computed tomography is a very well-known technique. Again, the prior art relates to methods and apparatus for practicing x-ray computed tomography. However, this technique also requires access to multiple sides of the region of interest because it is premised on the taking of multiple radiography measurements.
In recognition of this deficiency, a number of techniques have been developed that provide three-dimensional or depth information for objects located behind or embedded within an opaque material when the region of interest is accessible from only one side. One such technique is Compton backscatter tomography (CBT) as described by Zhu et al. in Measurement Science and Technology (1996).
In CBT, attenuation of radiation along a scattering path is measured and algorithms similar to those used in conventional x-ray computed tomography are applied to create a three-dimensional reconstruction of the entire region-of-interest.
As a variation on the general CBT technique, Faust (US 2004/0218714) teaches a method and apparatus that uses a coded aperture to image a buried landmine or improvised explosive device. The method produces slices at different depths within the region of interest because the size of the image projected through the coded aperture depends upon the depth at which the photons forming the image originated.
Another variation on CBT is taught by Shedlock, Meng, Sabri, Dugan and Jacobs (US 2011/0200172). This method and apparatus scans a fan-shaped beam across a region of interest and uses algebraic reconstruction or back projection techniques to generate an image. When the scanning is arranged so that a number of scan paths overlap, three-dimensional information about the object or medium can be obtained in the regions of overlap.
Yet another variation on CBT is taught by Annis (U.S. Pat. No. 7,620,150) in a system for backscatter imaging of objects at shallow depths, especially in human skin. The method involves scanning multiple beams of radiation across the region-of-interest and collecting the scattered radiation. Image contrast is created by differences in attenuation and density of the material along the beam path. By moving the source to different locations and using standard image combination techniques, the method can produce three-dimensional tomography images.
Another technique for obtaining depth information is to use x-ray photons having different energies. Higher energy photons penetrate more deeply into the object or medium under investigation. Therefore, scattered photons with low energy are more likely to have scattered from material at shallow depths, whereas scattered photons with high energy are more likely to have scattered from material at deeper depths.
Grodzins and Adams (U.S. Pat. No. 6,424,695) determine the depth of an object behind an opaque barrier using a source x-ray beam and two or more detectors located at different distances from the beam axis. In this arrangement, detectors furthest from the beam axis receive more scattering originating from deeper portions of the region of interest as a proportion of the total amount of scattering than do detectors closer to the beam axis. This occurs because the solid angle viewed by a far detector is large for deep portions and small for shallow portions, whereas the opposite is true for a close detector. The result is that closer detectors preferentially image shallow objects and farther detectors preferentially image deeper objects.
Morton (US 2012/0134473) uses a pulsed x-ray beam and measures the time delay between the outgoing pulse and when the scattered signal arrives at the detector. By scanning the beam across the region of interest, this technique creates a three-dimensional map of the surface of the object under investigation and works reasonably well for detecting weakly-scattering objects embedded within highly-scattering media.
In a rather different approach, Penny and Valentine (U.S. Pat. No. 8,314,394 and U.S. Pat. No. 8,426,822) create photons for imaging via positron-electron annihilation. This process creates two photons travelling in opposite directions. If one of these photons is detected near the annihilation event, then the direction of the second photon is known. When photons scattered within the region of interest are detected, they can be correlated with the outgoing photons so that the exact trajectory of each photon through the material is known.
Furthermore, multiply-scattered photons can be filtered out on the basis of their significantly lower energy. In this manner, the technique provides three-dimensional information about the scattering properties of the material.
Finally, Wood (U.S. Pat. No. 8,433,037) teaches an x-ray radar technique for generating three-dimensional information about a target object. Outgoing x-rays are created with a radio-frequency modulation using a radio-frequency modulated electron beam passed across a micro-channel plate. The modulation persists in the backscattered x-rays, and strikes a scintillator or the like to produce visible photons. The scintillation photons are then amplified by the micro-channel plate before striking the detector. The contrast on the detector represents the magnitude of the difference between the radio-frequency phase bias on the micro-channel plate and the radio-frequency modulated backscattered x-rays. By acquiring a subsequent frame with a 90° phase shift on the micro-channel plate, the technique generates three-dimensional information. The ratio of the two 90° phase-shifted frames gives a value proportional to the range, and calculating the arctangent of this ratio will provide reasonably precise range increments.
In view of the foregoing there is plainly a longstanding but currently unmet need for durable, flexible, time- and cost-effective methods and means of constructing two- and three-dimensional images of objects embedded within signal scattering media that overcome the shortcomings of the prior art and admit to meaningful evaluation of otherwise difficult to assess subject topographies.